The manufacturing industry is now widely employing two-degrees-of-freedom control devices having a feedback control system that provides second simulation torque signals and torque commands to, and a feedforward control system that provides first simulation torque signals to a machine system that comprises: a load machine such as a table or robot arm in a machine tool; a drive device such as a dc electric motor, an induction electric motor, a synchronous electric motor, an electromagnet, or a linear motor that drives the load machine; and a transmission mechanism that links the load machine and the drive device.
Examples of such devices are described in Japanese Patent Laid-Open No. 119402/1992 and Japanese Patent Laid-Open No. 138223/1992. FIG. 1 is a block diagram showing an example of a two-degrees-of-freedom control device of the prior art.
As shown in FIG. 1, the position control device of the prior art is provided with: motor 3, feedforward signal operational circuit 21, rotation detector 20, position control circuit 22, speed control circuit 23, and control means (torque control circuit) 24. Electric motor 3 drives load machine 1 by way of torque transfer mechanism 2. Feedforward signal operational circuit 21 receives rotation angle command signals of the electric motor θms from command generator 7 and, by means of a prescribed functional operation that includes at least two integrating operations, provides a simulation rotation angle signal θo, a simulation speed signal ωo, and a first simulation torque signal To as output. Rotation detector 20 detects the rotational speed and rotation angle of the electric motor. Position control circuit 22 supplies a first speed signal based on the simulation rotation angle signal θo and the actual rotation angle signal θm provided from the rotation detector 20. Speed control circuit 23 provides as output a second simulation torque signal T1 based on the simulation speed signal ωo, the first speed signal, and the actual speed signal ωm provided from the rotation detector 20. Control means 24 controls the torque of the electric motor 3 based on the first simulation torque signal T0 and the second simulation torque signal T1. This circuit configuration allows high-response position control performance.
When the control sampling time period of the feedforward operation becomes greater than the control sampling time period of the feedback operation, however, the difference in the sampling times causes a discrete modeling error even though the numerical model of the feedforward may match the object of control. As a result, deviation occurs between the actual rotation angle signal and the simulation rotation angle signal, and overshoot or vibration may therefore occur in the actual rotation angle signal.
It is an object of the present invention to provide an electric motor control device capable of realizing superior control performance in which deviation does not occur between the actual rotation angle signal and the simulation rotation angle signal even when the control sampling time period of the feedforward operation differs from the control sampling time period of the feedback operation.